Such devices are used to detect excessive loss of pressure, for example the oil pressure in a motor vehicle oil pump, or to detect excessive increase in pressure, for example in enclosures containing gas under high pressure A warning indication is then given to make it possible to take action to avoid the dangers which are the normal consequences of such variations in pressure beyond predetermined thresholds.
The state of the art may be illustrated by U.S. Pat. No. 3,064,094, by British Pat. No. 1 304 085 or by French Pat. No. 2 521 341. Reference can also be made to earlier devices shown in British Pat. No. 1 440 756, U.S. Pat. No. 3,121,145, and French Pat. No. 2 107 788.
Most existing devices include only one moving contact member, even though some applications require two detection thresholds: a first to give a warning; and a second to indicate that the situation has become dangerous and in general that operation must be stopped.
Present two-threshold devices are complicated in structure, require a large number of component parts, and are not always very reliable, mechanically or electrically.
A typical two-threshold device is described in French Pat. No. 2,513,313 which shows a pressure switch including an insulating housing bearing contact blades, and having a cavity which receives an insulating moving member supporting contact members. The device further includes a fixed conductive cap having a pusher passing therethrough, with one end of the pusher being in mechanical contact with a membrane whose opposite face is subjected to the action of a pressure. Resiliently deformable holding means urge the contact members against the fixed conductive cap, and said contact members co-operate with said cap to open or close an electric circuit including said tact blades as a function of the pressure acting on the membrane relative to two predetermined threshold pressures.
The structure of this prior device is complicated, and in particular the fixed conductive cap is in the form of a kettle drum having several openings therethrough, and the associated contact member has three branches. The component parts are located on both sides of the drum wall, thereby complicating the structure and the operation of the device, and also providing extreme positions in which one contact is closed while the other is open, which means that an external relay must be provided in order to obtain extreme positions in which both contacts are in the same configuration. Further, such complex structure reduces both the reliability and the accuracy of the device: in addition to the danger of the telescopic parts jamming, there is a risk of the contact disposed between the end of the insulating moving member and the wall of the fixed conductive drum being crushed in the event of a pulse of excess pressure (hammering); finally, since the active area of the membrane varies in the course of operation (pusher, then pusher plus moving member), a suitably stiff supporting spring must be provided, in spite of the reduced accuracy entailed thereby.
In the motor manufacturing industry, in particular, there is a need for monitoring and/or warning devices for use with hydraulic circuits for lubrication or for braking, and such devices should be reliable, accurate, and competitive in price.
An object of the invention is to provide a two-threshold pressure switch for monitoring and/or providing warning signals concerning abnormal pressure variations, which switch should be simple in structure and capable of long-term accuracy and reliability.
Another object of the invention is to provide a switch capable of being used both for abnormal pressure increases and for abnormal pressure drops.
Another object of the invention is to provide a pressure switch capable of directly replacing an existing switch without requiring complicated disassembly.